1. Field of the Invention
The present invention relates to a recording medium on which information is optically recorded or reproduced, such as an optical disc, an optical card and the like, and more particularly, to a recording medium which has a recording layer irradiated with a light beam for recording or reproducing information, and a recording and reproducing method and a recording and reproducing device therefor.
2. Description of the Related Art
In the optical disc such as DVD (Digital Versatile Disc) and the like, information signal is recorded as changes in the reflectivity and the recording and reproducing are conducted. For example, there is a re-recordable optical disc having a recording layer with uneven pre-grooves or lands provided as guide tracks, in which, by using the irradiation of light beam, pits or the like are recorded as a mark changed in the reflectivity different form that of the surrounding. In the recording and the reproducing to and from the optical disc, the light beam is converged to the guide track to form a light spot, while the light spot follows the guide track (with a tracking servo control), light reflected and return from mark on the guide track is detected to reproduce the information signals.
To realize an information recording in a high density to the optical disc, attempts to minimize the light spot and to shorten each mark length and each track pitch still more. However, the narrowing of track pitch decreases the output level of the tracking error signal particularly a radial push-pull signal. This is a problem of a low signal level insufficient for the tracking servo control to be impracticable, namely one of obstructions to a high density information recording.
In addition, the hologram has drawn attention because of its ability to record two-dimensional data signals at a high density, in order to increase the density of recording information. The hologram is characterized by volumetrically recording a wavefront of light which carries recording information on a hologram recording medium made of a photosensitive material such as a photo-refractive material as a change in refractive index. For example, a recording and reproducing system which utilizes the hologram recording medium as a disc (hologram disc) has been developed (see Laid-open Japanese Patent Application Kokai No. 11-311937).
FIG. 1 shows part of a hologram disc of an air gap type. This hologram disc is comprised of a reflective substrate 221, a transparent substrate 222 facing a reflecting surface of the reflective substrate, and a hologram layer 225 combined with the transparent substrate 222 on the reflective substrate 221 side. The air gap with a predetermined thickness is disposed between the reflecting surface of the reflective substrate 221 and the hologram layer 225. In the hologram disc, pre-grooves are formed on the reflecting surface of the reflective substrate 221.
As shown in FIG. 2, the reflective film of the hologram disc has a reflective surface which is formed with servo areas 6 at predetermined angular intervals. There is a data area 7 in a sector area every between adjacent servo areas 6. A groove 201 (pre-groove) is formed per each track in the servo area 6 for the tracking servo control. On the other hand, the data area 7 has no groove. The conventional system forces the light spot LS to follow the pre-groove of the servo area 6 in the tracking servo control of the recording and reproducing of the hologram disc. During the passage of the light spot LS on the data area 6, the tracking servo control is not preformed, and the objective lens is fixed in the pickup device.
In the hologram recording system, a hologram disc with reference light which is converged on the reflective film through the recording layer as a spot, and the reference light reflected by the reflective film diverges to pass through the recording layer, and simultaneously, information light, which carries information to be recorded, is passed through the recording layer. In this way, in the recording layer, the reflected reference light interferes with the information light to form an interference pattern to volumetrically record hologram within the recording layer. The holograms of the interference pattern are recorded in the recording layer adjacent to each other, overlapping in sequence. Then, the reference light is irradiated to detect and demodulate reproduced light restored from each hologram to reproduce recorded information.
In general, the track pitch may be decided on the basis of the multiplicity of hologram. Therefore, the track pitch is wider in comparison with the spot diameter of the light beam that is servo controlled for the recording and reproducing of hologram. Thus, FIG. 2 shows an exemplary transfer of a light spot LS during the recording or reproducing of hologram. When the light spot LS transfers from the one track to the adjacent track in the radial direction, the tracking error signal is not output at all. Therefore, the light spot LS is derailed from the track after which the light spot should chase due to external interference, perturbations and the like. It is difficult to perform the tracking servo control of the objective lens.
On the other hand, if the track pitch is set to meet with the spot diameter of the light spot, then the multiplicity of hologram will increase, so that the light spot may erase adjacent holograms and signals, otherwise a cross-talk may occur during the reading. In addition, the recording medium having a narrow track pitch may bring about an unexpected situation that S/N of read out signal is deteriorated due to the influence of diffraction caused by the narrow guide track structure.